page_title: Van Deemter equation
section_title: Plate count
hierarchical_section_title: Van Deemter equation / Plate count
caption_attribution_description: Chromatogram showing two separate peaks, resolution = 2.5
context_page_description: The van Deemter equation in chromatography, named for Jan van Deemter, relates the variance per unit length of a separation column to the linear mobile phase velocity by considering physical, kinetic, and thermodynamic properties of a separation. These properties include pathways within the column, diffusion, and mass transfer kinetics between stationary and mobile phases. In liquid chromatography, the mobile phase velocity is taken as the exit velocity, that is, the ratio of the flow rate in ml/second to the cross-sectional area of the ‘column-exit flow path.’ For a packed column, the cross-sectional area of the column exit flow path is usually taken as 0.6 times the cross-sectional area of the column. Alternatively, the linear velocity can be taken as the ratio of the column length to the dead time. If the mobile phase is a gas, then the pressure correction must be applied. The variance per unit length of the column is taken as the ratio of the column length to the column efficiency in theoretical plates.
context_section_description: The plate height given as:
  with  the column length and  the number of theoretical plates can be estimated from a chromatogram by analysis of the retention time  for each component and its standard deviation  as a measure for peak width, provided that the elution curve represents a Gaussian curve.
  In this case the plate count is given by:
  By using the more practical peak width at half height  the equation is:
  or with the width at the base of the peak:
caption: Two well resolved peaks in a chromatogram
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